Consolidation of non woven fibrous fabrics can be achieved by heating or by hot calendering said fabrics. Calendering a sheet at high temperature usually increases its strength and lowers its porosity while heating alone does not prove to be sufficient to attain the same physical properties. Nevertheless, the high porosity required for certain applications is still obtained by heating alone. The fabric exhibits enhanced properties if both temperature and pressure are applied.
Due to their properties, aramid fibers and aramid fibrids are commonly incorporated into fabrics in order to prepare strong, high temperature resistant supports that show good electrical insulation aptitude.
In U.S. Pat. No. 5,667,900, an aramid support with high surface smoothness is described. This paper is prepared by laminating layers containing meta-aramid fibrids and aramid flocks. The nature and properties of the aramid polymer comprised in this paper make it particularly suitable for being used as electrical insulation paper, or heat-resistant paper.
A laminate containing para-aramid fibers is taught in U.S. Pat. No. 6,558,512. This laminate exhibits high strength, reduced thickness, and light weight. This non woven fabric contains para-aramid fibers as well as a thermosetting resin.
The laminate disclosed in U.S. Pat. No. 5,948,543 essentially consists of a non woven fabric comprising para-aramid and meta-aramid fibers that are adhered to each other by a resin binder. Thermal binding between meta-aramid fibers, and between meta-aramid fibers and para-aramid fibers is further achieved by hot calendering.
Such papers are preferably entirely made of aramid fibers since the presence of inorganic fibers may lead to an increase in porosity while good electrical insulation certainly requires low porosity.
On the other hand, the paper used to make honeycombs can contain, beside aramid, cellulose, glass fibers or carbon fibers without negatively affecting its properties for this particular application.
In fact, the composition of a paper is directly related to the application it is intended for. For instance, the aramid based Nomex® is used for electrical insulation, it is manufactured by mixing poly(metaphenylene isophthalamide) (=meta-aramid polymers) flocks and fibrids and then subjecting the mixture to hot-press calendering.
Aramid fabrics are made of high temperature resistant fibers, usually aramid fibers and aramid fibrids. As already mentioned, they can be combined with other fibers such as cellulose.
These fabrics containing synthetic fibers such as aramid fibers are strengthened after being calendered. A resin binder is usually required; however, it does not allow to completely retain the original properties of the aramid fibers.
The Applicant has developed a consolidated support containing synthetic fibers. Its stiffness and strength are increased by more than 30% as compared to standard supports of the prior art. Moreover, the properties of the synthetic fibers are not altered during the manufacturing of this fibrous support.